Piezoelectric actuators usually consist of a number of piezoelectric elements arranged in a stack. Each of these elements in its turn consists of a piezoceramic layer on both sides of which metallic electrodes are provided. If a voltage is applied to these electrodes, the piezoceramic layer reacts with a lattice distortion, which leads along a main axis to a usable length expansion. Since this in its turn amounts to less than 2 thousandth of the layer thickness along the main axis, to achieve the desired absolute length expansion a correspondingly greater layer thickness of active piezoceramic must be provided. However, as the layer thickness of the piezoceramic layer of an individual piezoelectric element increases, the voltage required for the piezoelectric element to respond also increases. To keep this within manageable limits, the thicknesses of individual elements usually lie between 20 and 200 μm. A multilayer piezoelectric actuator must therefore have a corresponding number of individual elements or layers for a desired length expansion.
Known multilayer piezoelectric actuators thus generally consist of numerous individual layers in total. To produce said actuators, piezoceramic layers are arranged alternately with electrode material to form a stack and are laminated and sintered together to form a monolithic assembly. Such a method is known for example from European patent EP 0 894 340 B1. In this patent an electrode material is printed onto piezoceramic green tapes. The printing is undertaken in accordance with a pattern, which includes printed areas and non-printed areas left free. The electrode layers are stacked alternately such that a surface printed with electrode material in the next adjacent second electrode layer is arranged over each unprinted area in a first electrode layer. The alternating arrangement means that every second electrode layer covers the same area in relation to its pattern of electrodes. In the intermediate electrode layers, which also cover the same area, the unprinted areas left free are offset diagonally.
In this approach in accordance with the prior art however the disadvantageous fact has emerged that during sintering, an internal pressure and volume equalization occurs, with for example the unprinted areas left free in the electrode pattern being completely filled by ceramic material of the adjacent ceramic layers. During an electrical contacting of the electrode layers all ceramic layers undergo an expansion, with the exception of the corner areas comprising the unprinted areas, since no effective electrical field strength can be built up in these two diagonally-opposite areas. By contrast with the rest of the ceramic areas, these corner areas thus do not experience any expansion on activation, which means that mechanical stresses occur between the active areas and the inactive areas. These mechanical stresses cause cracks to form which can start from the inactive corner areas and extend into the active ceramic areas. Such cracks can propagate in an unchecked way through the ceramic in a longitudinal direction as well and considerably reduce the service life of the piezoelectric actuator.
The process of integrating stress-relieving layers into the stack in accordance with a predetermined alternating sequence of ceramic and electrode layers in order to create predetermined separation areas to reduce mechanical stresses is known to the inventor.
In this approach however the fact that the incorporation of additional stress-relieving layers involves an additional production effort and additional production costs has proved disadvantageous. In addition these stress-relieving layers disadvantageously increase the size of the stack.
Further measures are also known to the inventor for reducing mechanical stresses in the piezoelectric actuator, which however do not prevent the formation of cracks in general but merely a formation of cracks in a longitudinal direction of the piezoelectric actuator, which means that the function of the piezoelectric actuator is largely preserved. Despite this, there is also the risk with these measures of unchecked crack propagation resulting from residual transverse cracks, which reduces the power of the piezoelectric actuator or can possibly even destroy it.